diff: add ability to insert additional headers for paths

diff: add ability to insert additional headers for paths

When additional headers are provided, we need to
  * add diff_filepairs to diff_queued_diff for each paths in the
    additional headers map which, unless that path is part of
    another diff_filepair already found in diff_queued_diff
  * format the headers (colorization, line_prefix for --graph)
  * make sure the various codepaths that attempt to return early
    if there are "no changes" take into account the headers that
    need to be shown.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>